The present invention relates to an image reading device for use with a digital copying machine, for example, and more particularly to an image reading device with a shading correction device.
In the digital copying machine, an original is placed on a platen glass located on the top of the base machine. Pictorial information on the original is read by an image sensor of an image reading device and converted into image signals. The image signals are appropriately processed, and are outputted in the form of a reproduced picture on paper by means of a Xerography unit or a thermal transfer unit contained in the base machine.
An original is linearly illuminated by a line light source, such as a fluorescent lamp. Reflected light from the original is linearly read by a line sensor. It is ideal that light from the light source be uniformly distributed over the entire width of the original. In actual cases, however, the distribution is not uniform, because variations in the light light emitting characteristics of the fluorescent lamps inevitably exist, and optical paths are changed. Nonuniform distribution of light causes shading, i.e., irregularity of optical density appearing on the read image.
It is, for this reason that a shading correction unit is used for the image reading device of this type. To make a shading correction, a reference white color plate is read before reading an original image. Levels of image signals derived from the image sensor when the white color plate is read are stored as correction data. The correction data are used as correcting levels for the image signals gathered when an original is actually read. In this way, the shading of an output image is corrected.
The image reading device with the shading correction typically comes in two varieties; a fixed original type and a moving original type. For reading an image on a large size original, the moving original type is introduced normally as that of the image reading device. The reason for this is that, if the image reading device of the fixed original type is introduced, the largest size of the original that can be handled is limited by an area of the platen glass.
In the image reading device of the moving original type, the platen glass used is rectangular in shape, and wider than the original but is shorter than the original as viewed in a direction carrying the original. The original is nipped between the platen glass and the platen roller. An image on the original is read through the platen glass, while being moved by the platen roller. Many conventional shading correction units adaptable for the image reading device of the moving original type are known. In one of those known units, a white color member is used for the platen roller, and the roller per se is a reference white color plate.
This type of the shading correction unit involves some problems to be solved. The material quality required for the platen roller that must reliably carry the original, while satisfactorily pressing the original against the platen glass, does not satisfy a requirement for the reference white color plate. The best compromise that has been taken for the contradictory requirements is that either of the functions of the platen roller and the reference white color plate is sacrificed; when the platen roller-satisfactorily functions, the reference white color plate functions unsatisfactorily, and vice versa. Thus, it is impossible to simultaneously obtain satisfactory mechanical characteristics to carry the original and satisfactory optical characteristics for shading correction.
The platen roller rotates while constantly pressing the original against the platen glass. Therefore, it is apt to become contaminated. The contaminated platen roller unsatisfactorily exercises its function.
To cope with the problem, there is a proposal in which a reference white color plate 33, as shown in FIG. 7, is disposed near the image reading area on the upper surface of a platen glass 22 with which a platen roller 31 is to engage. The white color plate 33 is covered with a protecting member 34. In operation, an optical path for reading image (as indicated by a solid line) or another optical path for reading reference white color (as indicated by a broken line) is selected by a movable mirror 35. This proposal needs an additional optical system near the platen glass, and a space for the optical system installation. In this respect, a design freedom for the image reading device is limited. The protecting member 34, which is for the white color plate, is obstructive in transporting the original That is, it may cause improper original carriage or a variation in the original carrying speed.
To solve this problem, the white color plate may be placed on the underside of the platen glass 32. In this case, the white color plate is located out of the focused surface, and the platen glass is not involved in the image reading operation. Therefore, the conditions for reading the white color plate are different from those for the image reading. The difference of the conditions leads to inexact image reading. An additional, disadvantage involved is that heat from a heat source often changes the white color plate to a yellowish color.
There is another approach to solve the platen roller contamination problem, as shown in FIGS. 8(a) and 8(b), the platen glass 32, together with the white plate 32, is movable a small distance in a carrying direction of the original. Before reading an original image, the white color plate, as shown in FIG. 8(b), is read by moving the platen glass 32 in the arrowhead direction so that the white color plate 33 comes right under the platen roller 31. To read an original, the platen glass 32 is moved so that the white color plate 33 retracts the optical path, as shown in FIG. 8(a).
The approach requires a complicated mechanism to move the platen glass 32 and a space for installing it. Where the white color plate 33 is disposed on the upper surface of the platen glass 32 as shown in FIG. 8(b), the platen roller possibly rides on the protecting member 34 covering the white color plate 33. If it rides on the protecting member, the platen roller 31 must be positioned correctly.